Field of the Invention
The present invention relates to a circuit for adjusting the sensitivity of an optical character reader for optically reading information, the optical character reader applying light to the information by a light source comprising a plurality of light emitting materials, for example, an LED array comprising a plurality of LEDs. The invention also relates to a method of adjusting the sensitivity of the optical character reader.
Description of the Related Art
Conventionally, a general optical character reader optically reading information is operated such that an LED array (light source) comprising a plurality of LEDs (light emitting materials) applies light to a medium in which information (for example, a bar code) is recorded and a line sensor reads the reflected light from the medium, thereby reading the information. The following is used as a line sensor for the optical character reader: a CCD line sensor comprising a photoelectric conversion section including a plurality of photodiodes; a charge storage section including PN junctions of the respective photodiodes; and a charge transfer section including CCDs (Charge Coupled Devices) the number of which accords with the number of photodiodes. The photoelectric conversion section converts the incident light coming from the outside into electric charges the amount of which accords with the amount of received light. The charge storage section temporarily stores the electric charges obtained in the photoelectric conversion section and feeds them to the charge transfer section at one time by applying a start pulse. The charge transfer section transfers the charges by stages every time a clock pulse is applied so as to transmit them from the line sensor as a chronological series analog signal.
The amplitude of the series analog signal transmitted from the line sensor is obtained in the photoelectric conversion section. It relies on the amount of charges stored in the charge storage section. The amount of charges further relies on the time interval between the application of one start pulse and that of the next start pulse, that is, the storage time.
Although the sensitivity of receiving the reflected light by the line sensor is substantially uniform, the components other than the line sensor have varied characteristics, and more specifically, the luminances of the LEDs forming the LED array vary, thus causing variations in the amounts of light applied to the medium even though the light emitting time is uniform regardless of the LEDs. In the above-mentioned optical character reader, the discrepancy between amounts of reflected light to the line sensor directly causes the discrepancy between amounts of charges. The optical character reader determines the information on the basis of the discrepancy between amounts of charges. Thus, the optical character reader might mistakingly determines the discrepancy between amounts of light due to variations in the luminances of the LEDs as information on the medium.
After the series analog signal is amplified, it is binary-coded. When the amplitude is too large, the series analog signal is saturated while being amplified, thus failing to be accurately binary-coded. On the other hand, when the amplitude is too small, the series analog signal cannot obtain a suitable threshold level when it is binary-coded, thus also failing to be accurately binary-coded.
Hence, in order to accurately execute binary-coded processing on the series analog signal, it is necessary to make the amplitude of the series analog signal substantially constant. In particular, when there are variations in brightness of the LEDs forming the LED array, it is necessary to correct such variations. Conventionally, an LED array is formed by the following method. A plurality of LEDs are first classified into different luminance levels, and the LEDs with the same luminance are used for forming the LED array. However, such a method makes the formation of the LED array extremely complicated.
Also, although the luminance of the LEDs forming the LED array varies due to changes in temperature or lapse of time, a discrepancy between the amounts of light applied to a medium due to the variations in the luminance can no longer be corrected.